Cervical disc replacement

ABSTRACT

An apparatus for replacing at least a portion of an intervertebral disc in a spinal column includes: a first member having a first vertebral contact surface for engagement with an endplate of a first vertebral bone in the spinal column, and having a first articulation surface that is defined by a plurality of concave arcs each having a respective radius of curvature about a corresponding axis substantially perpendicular to an anterior-posterior plane of the spinal column, and a plurality of convex arcs each having a respective radius of curvature about a corresponding axis substantially perpendicular to a lateral plane of the spinal column; and a second member having a second vertebral contact surface for engagement with an endplate of a second vertebral bone in the spinal column, and having a second articulation surface that is defined by a plurality of convex arcs each having a radius of curvature about a corresponding axis substantially perpendicular to the anterior-posterior plane of the spinal column, and a plurality of concave arcs each having a radius of curvature about a corresponding axis substantially perpendicular to the lateral plane of the spinal column, wherein: an intervertebral disc space is defined substantially between the first and second endplates of the first and second vertebral bones, and the radii of curvature of the first and second articulation surfaces are sized such that the first and second articulation surfaces engage one another when the first and second members are disposed in the intervertebral disc space to enable the first and second vertebral bones to articulate in at least one of flexion, extension and lateral bending.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This is a continuation application of U.S. patent applicationSer. No. 10/382,702, filed Mar. 6, 2003, entitled CERVICAL DISCREPLACEMENT, the entire disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] The present invention is directed to a cervical joint replacementimplant and more particularly to a cervical intervertebral discprosthesis having opposing constant radii saddle shaped articulatingsurfaces.

[0003] The structure of the intervertebral disc disposed between thecervical bones in the human spine comprises a peripheral fibrous shroud(the annulus) which circumscribes a spheroid of flexibly deformablematerial (the nucleus). The nucleus comprises a hydrophilic, elastomericcartilaginous substance that cushions and supports the separationbetween the bones while also permitting articulation of the twovertebral bones relative to one another to the extent such articulationis allowed by the other soft tissue and bony structures surrounding thedisc. The additional bony structures that define pathways of motion invarious modes include the posterior joints (the facets) and the lateralintervertebral joints (the unco-vertebral joints). Soft tissuecomponents, such as ligaments and tendons, constrain the overallsegmental motion as well.

[0004] Traumatic, genetic, and long term wearing phenomena contribute tothe degeneration of the nucleus in the human spine. This degeneration ofthis critical disc material, from the hydrated, elastomeric materialthat supports the separation and flexibility of the vertebral bones, toa flattened and inflexible state, has profound effects on the mobility(instability and limited ranges of appropriate motion) of the segment,and can cause significant pain to the individual suffering from thecondition. Although the specific causes of pain in patients sufferingfrom degenerative disc disease of the cervical spine have not beendefinitively established, it has been recognized that pain may be theresult of neurological implications (nerve fibers being compressed)and/or the subsequent degeneration of the surrounding tissues (thearthritic degeneration of the facet joints) as a result of their beingoverloaded.

[0005] Traditionally, the treatment of choice for physicians caring forpatients who suffer from significant degeneration of the cervicalintervertebral disc is to remove some, or all, of the damaged disc. Ininstances in which a sufficient portion of the intervertebral discmaterial is removed, or in which much of the necessary spacing betweenthe vertebrae has been lost (significant subsidence), restoration of theintervertebral separation is required.

[0006] Unfortunately, until the advent of spine arthroplasty devices,the only methods known to surgeons to maintain the necessary disc heightnecessitated the immobilization of the segment. Immobilization isgenerally achieved by attaching metal plates to the anterior orposterior elements of the cervical spine, and the insertion of someosteoconductive material (autograft, allograft, or other porousmaterial) between the adjacent vertebrae of the segment. Thisimmobilization and insertion of osteoconductive material has beenutilized in pursuit of a fusion of the bones, which is a procedurecarried out on tens of thousands of pain suffering patients per year.

[0007] This sacrifice of mobility at the immobilized, or fused, segment,however, is not without consequences. It was traditionally held that thepatient's surrounding joint segments would accommodate any additionalarticulation demanded of them during normal motion by virtue of thefused segment's immobility. While this is true over the short-term(provided only one, or at most two, segments have been fused), theeffects of this increased range of articulation demanded of theseadjacent segments has recently become a concern. Specifically, anincrease in the frequency of returning patients who suffer fromdegeneration at adjacent levels has been reported.

[0008] Whether this increase in adjacent level deterioration is trulyassociated with rigid fusion, or if it is simply a matter of theindividual patient's predisposition to degeneration is unknown. Eitherway, however, it is clear that a progressive fusion of a long sequenceof vertebrae is undesirable from the perspective of the patient'squality of life as well as from the perspective of pushing a patient toundergo multiple operative procedures.

[0009] While spine arthroplasty has been developing in theory over thepast several decades, and has even seen a number of early attempts inthe lumbar spine show promising results, it is only recently thatarthoplasty of the spine has become a truly realizable promise. Thefield of spine arthroplasty has several classes of devices. The mostpopular among these are: (a) the nucleus replacements, which arecharacterized by a flexible container filled with an elastomericmaterial that can mimic the healthy nucleus; and (b) the total discreplacements, which are designed with rigid endplates which house amechanical articulating structure that attempts to mimic and promote thehealthy segmental motion.

[0010] Among these solutions, the total disc replacements have begun tobe regarded as the most probable long-term treatments for patientshaving moderate to severe lumbar disc degeneration. In the cervicalspine, it is likely that these mechanical solutions will also become thetreatment of choice. At present, there are two devices being testedclinically in humans for the indication of cervical disc degeneration.The first of these is the Bryan disc, disclosed in part in U.S. Pat. No.6,001,130. The Bryan disc is comprised of a resilient nucleus bodydisposed in between concaval-covex upper and lower elements that retainthe nucleus between adjacent vertebral bodies in the spine. Theconcaval-convex elements are L-shaped supports that have anterior wingsthat accept bones screws for securing to the adjacent vertebral bodies.

[0011] The second of these devices being clinically tested is theBristol disc, disclosed substantially in U.S. Pat. No. 6,113,637. TheBristol disc is comprised of two L-shaped elements, with correspondingones of the legs of each element being interposed between the vertebraeand in opposition to one another. The other of the two legs are disposedoutside of the intervertebral space and include screw holes throughwhich the elements may be secured to the corresponding vertebra; thesuperior element being secured to the upper vertebral body and theinferior element being attached to the lower vertebral body. Theopposing portions of each of the elements comprise the articulatingsurfaces that include an elliptical channel formed in the lower elementand a convex hemispherical structure disposed in the channel.

[0012] As is evident from the above descriptions, the centers ofrotation for both of these devices, which are being clinically tested inhuman subjects, is disposed at some point in the disc space. Moreparticularly with respect to the Bryan disc, the center of rotation ismaintained at a central portion of the nucleus, and hence in the centerof the disc space. The Bristol disc, as a function of its elongatedchannel (its elongated axis being oriented along the anterior toposterior direction), has a moving center of rotation which is, at alltimes maintained within the disc space at the rotational center of thehemispherical ball (near the top of the upper element).

[0013] Other aspects, features and advantages of the present inventionnot already evident will become evident from the description hereintaken in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

[0014] The present invention provides an articulating joint implant thatincludes a pair of opposing upper and lower elements having nestedarticulation surfaces providing a center of rotation of the implantabove the adjacent vertebral body endplate surfaces in one mode ofmotion (e.g., lateral bending) and a center of rotation of the implantbelow those surfaces in another mode of motion (e.g.,flexion/extension), and that further permit axial rotation of theopposing elements relative to one another (for example, about an axissuch as, for example, a longitudinal axis, for example, of the spinalcolumn)) through a range of angles without causing them to move indirections that are directed away from one another (for example, inopposing directions along the axis of axial rotation) within that range.In preferred embodiments, the articulation surfaces further cause suchopposite (or otherwise directed away from one another) movement of theopposing elements beyond that range.

[0015] More particularly, the present invention contemplates that withregard to the cervical anatomy, a device that maintains a center ofrotation, moving or otherwise, within the disc space is inappropriateand fails to properly support healthy motion. Specifically, the cervicaljoint comprises five separate articulating elements: the facet joints inthe posterior of the segment; the lateral unco-vertebral joints; and thenucleus in the intervertebral space. It is contemplated by the presentinvention that a track defined by the cervical facets falls along theplanes between the inferior surface of the upper facets and the superiorsurface of the lower facets, and that this plane extends upwardly andforward, forcing the overall joint to pivot around a center of rotationthat resides in the lower vertebral bone in flexion/extensionarticulation.

[0016] Conversely, it is contemplated by the present invention that inlateral bending the unco-vertebral joints influence the track of motion.Specifically, the unco-vertebral joints are formed at the lateral edgesof the intervertebral space and are defined by a pair of upwardlyextending surfaces of the inferior vertebral endplate and thecorresponding surfaces of the superior bony endplate. It is contemplatedby the present invention that this U-shaped configuration guides thesegment into a rotation about a center within the superior vertebralbone during lateral bending.

[0017] Finally, it is contemplated by the present invention that duringaxial rotation of the adjacent vertebral bones of the cervical segmentrelative to one another about the longitudinal axis of the spinalcolumn, the opposing bones do not simply axially rotate relative to oneanother for more than a few degrees, but rather follow the coupledinfluences of the unco-vertebral joints and the nucleus, and that thiscoupled motion vertically separates the opposing bones of the facetjoints as the rotation continues, thus freeing the bones to rotatefarther that would otherwise be permitted if the facets locked together(as is often seen as a symptom of degenerative cervical disease).

[0018] Both the Bryan and Bristol discs described above do providedistraction and maintenance of intervertebral disc height, and therebyprovide immediate and short-term relief from pain. However, it should beunderstood, in light of the above described anatomy as contemplated bythe present invention, that neither provides for proper anatomicalmotion because their respective centers of rotation are located withinthe disc space. Thus, neither will afford significant motionpreservation, and patients with these devices implanted in their neckswill find no significant mobility at the implanted segment. This maylead to spontaneous fusions, long term facet deterioration, and/oraccelerated adjacent level degeneration.

[0019] The constraints placed on the prosthesis by the above-describedanatomy are considerable. To provide an implant having a pair ofarticulation surfaces that provide a center of rotation of the implantabove the surfaces in one mode of motion (lateral bending) and a centerof rotation of the implant below the surfaces in another mode of motion(flexion/extension), that further permit the surfaces to axially rotaterelative to one another about a longitudinal axis of the spinal columnthrough a range of angles without causing movement of the surfaces inopposing directions along the longitudinal axis of the spinal column,and that further cause such movement (and accordingly a verticalseparation of the facet joints) beyond that range is a difficultengineering task. The present invention contemplates that a saddle jointprovides a geometric approach to the task.

[0020] The solution to this problem, however, is not open to just anysaddle joint configuration. U.S. Pat. Nos. 5,405,400 and 5,645,605describe saddle joints utilized for prosthetic thumb joints. Moreparticularly, U.S. Pat. No. 5,405,400 (“Linscheid”) discloses anartificial thumb joint comprising a pair of surfaces that are nestinghyperbolic paraboloids. A hyperbolic paraboloid is a surface defined bya first specific geometric form (the hyperbola) that is swept along asecond geometric form (the parabola) that is perpendicular to the firstform, and which first and second forms are opposite in the direction oftheir convexities. A common feature of both hyperbolae and parabolas isthat neither has a constant radius of curvature along its extent.Constant radii of curvature are necessary for a pair of surfaces tosmoothly flow over one another. Accordingly, the nesting hyperbolicparaboloids set forth in the reference are, therefore, not capable ofany smooth articulation. Any attempted articulation causes the twosurfaces to immediately move in opposing directions. Stated more simply,nesting hyperbolic paraboloids have continuously changing centers ofrotation (by the nature of the geometric forms selected). The presentinvention contemplates that the cervical joint anatomy enables smootharticulation in two modes of motion (lateral bending andflexion/extension), and also smooth relative axial rotation about thelongitudinal axis of the spinal column through a small range of angles.It is understood by the present invention that the vertical separationmotion of the natural cervical joint does not occur immediately, butrather occurs only outside that small angular range of relative axialrotation. Thus, the present invention contemplates that the nestinghyperbolic paraboloids disclosed by Linscheid are inappropriate for usein the cervical joint.

[0021] U.S. Pat. No. 5,645,605 (“Klawitter”) discloses an alternate formof a saddle surface, again for use in an artificial thumb joint, thatcomprises a pair of nesting toroidal surfaces. Toroidal surfaces aredefined by an arc of one circle being swept along an arc of another,again having opposing convexities. As circles have constant radii ofcurvatures, it is possible with these surfaces to have smooth motion intwo perpendicular planes. More particularly, if the corresponding radiiof curvature are approximately equivalent, the two surfaces may nest andarticulate in flexion/extension and lateral bending smoothly, withoutcausing the surfaces to move in opposing directions upon an attemptedarticulation. However, Klawitter teaches that these toroidal surfacesshould have the same radii of curvature, inasmuch as it is a necessitythat axial rotational motion of the joint be inhibited, or if it ispermitted to occur, it should cause an immediate axial distraction ofthe joint. As explained above with regard to the saddle joint inLinscheid, this elimination of the capacity of the surfaces to axiallyrotate relative to one another for even a small range of angles preventssuch a design from being effectively used in a cervical discapplication.

[0022] In order for two nesting toroidal saddles to rotate within thesame plane, each of the concave radii of the surfaces must be greaterthan the radius of its nested convex surface. An artificial cervicaljoint that provides a center of rotation in the vertebral bone belowduring flexion/extension and in the vertebral bone above during lateralbending and has the capacity to axially rotate within a small range ofangles prior to causing oppositely directed movement of the surfacesrequires nesting surfaces with such a configuration.

[0023] The present invention, therefore, provides an articulating jointimplant for use in the cervical spine, including a first (e.g., upper)element and a second (e.g., lower) element, each having an outwardlyfacing vertebral body contact surface, and each having an inwardlyfacing articulation surface. The elements are disposed with thearticulation surfaces nested against one another, and the vertebral bodycontact surfaces facing away from one another. When the implant isdisposed in an intervertebral disc space in a cervical spine, in thisconfiguration and with the vertebral body contact surfaces engaged withrespective adjacent vertebral body endplates, the implant enables theadjacent vertebral bones to move relative to one another in accordancewith proper anatomical motion.

[0024] Preferably, each of the elements has at least one long-termfixation structure (e.g., a flange) having at least one feature (e.g., athrough hole) for securing the element to an adjacent vertebral body.For example, the upper element has an anterior flange that extendsupwardly and has two through holes, each of which accepts a bone screw.And, for example, the lower element has an anterior flange that extendsdownwardly and has one through hole that accepts a bone screw. Furtherpreferably, each of the elements has at least one short-term fixationstructure (e.g., spikes on the outwardly directed vertebral body contactsurface) for securing the element to an adjacent vertebral bodyendplate.

[0025] Further preferably, each of the outwardly directed vertebral bodycontact surfaces is shaped to conform to the endplate of an adjacentvertebral body against which it is to be positioned. For example,vertebral body contact surface of the upper element is curvate (to matchthe anatomy of the superior vertebral body endplate) and the vertebralbody contact surface of the lower element is flat (to match the anatomyof the inferior vertebral body endplate). Further preferably, eachvertebral body contact surface has an osteoinductive or osteoconductivefeature, such as, for example, porous or rough areas.

[0026] The longitudinally inwardly directed articulation surface of theupper element forms a constant radii saddle-shaped articulation surface.More particularly, the saddle surface is defined by a concave arc thatis swept perpendicular to and along a convex arc. The articulationsurface has a cross-section in one plane that forms a concave arc, and across-section in another plane (perpendicular to that plane) that formsa convex arc. The concave arc has a respective constant radius ofcurvature about an axis perpendicular to the one plane. The convex archas a respective constant radius of curvature about an axisperpendicular to the other plane.

[0027] In a preferred embodiment, the concave arc has a constant radiusof curvature A about an axis perpendicular to the anterior-posteriorplane, and the convex arc has a constant radius of curvature B about anaxis perpendicular to the lateral plane. Preferably, radius A is lessthan radius B.

[0028] The longitudinally inwardly directed articulation surface of thelower element also forms a constant radii saddle-shaped articulationsurface. More particularly, the saddle surface is defined by a convexarc that is swept perpendicular to and along a concave arc. Thearticulation surface has a cross-section in one plane that forms aconvex arc, and a cross-section in another plane (perpendicular to thatplane) that forms a concave arc. The convex arc has a respectiveconstant radius of curvature about an axis perpendicular to the oneplane. The concave arc has a respective constant radius of curvatureabout an axis perpendicular to the other plane.

[0029] In a preferred embodiment, the convex arc has a constant radiusof curvature C about an axis perpendicular to the anterior-posteriorplane, and the concave arc has a constant radius of curvature D about anaxis perpendicular to the lateral plane. Preferably, radius C is lessthan radius D.

[0030] The constant radii saddle shaped articulation surfaces areconfigured and sized to be nestable against one another andarticulatable against one another, to enable adjacent vertebral bones(against which the upper and lower elements are respectively disposed inthe intervertebral space) to articulate in flexion, extension, andlateral bending. More particularly, the artificial disc implant of thepresent invention is assembled by disposing the upper and lower elementssuch that the vertebral body contact surfaces are directed away from oneanother, and the articulation surfaces are nested against one anothersuch that the concave arcs accommodates the convex arcs.

[0031] Accordingly, movement of the adjacent vertebral bones relative toone another is permitted by the movement of the upper and lower elementsrelative to one another. In flexion and extension, the concave arcs ofthe upper element ride on the convex arcs of the lower element about acenter of rotation below the articulation surfaces. In lateral bending,the concave arcs of the lower element ride on the convex arcs of theupper element about a center of rotation above the articulationsurfaces. During these articulations, the elements are maintained atconstant relative distraction positions, i.e, the elements do not movein directions that are directed away from one another (for example, donot move in opposing axial directions from one another (e.g., along alongitudinal axis of the spine)). Accordingly, the present inventionprovides a pair of articulation surfaces that have a center of rotationabove the surfaces in one mode of motion (lateral bending), and belowthe surfaces in another (flexion/extension), consistent in these regardswith a natural cervical intervertebral joint. Preferably, thearticulation surfaces are sized and configured so that the respectiveranges of angles through which flexion/extension and lateral bending canbe experienced are equal to or greater than the respective normalphysiologic ranges for such movements in the cervical spine.

[0032] It is preferable that, in addition to the flexion, extension, andlateral bending motions described above, the adjacent vertebral bones bepermitted by the artificial disc implant to axially rotate relative toone another (e.g., about the longitudinal axis of the spinal column),through a small range of angles, without moving in opposite (orotherwise directed away from one another) directions (e.g., along thelongitudinal axis) within that range, and then to engage in suchopposite (or otherwise directed away from one another) movement oncethat range is exceeded. Preferably, the articulation surfaces 204, 304are accordingly configured and sized to permit such movements. In apreferred configuration, the constant radius of curvature A is largerthan the constant radius of curvature C, and the constant radius ofcurvature D is larger than the constant radius of curvature B. Becauseof the space, afforded by the differing radii, at the edges of thearticulation surfaces, the upper and lower elements are able to axiallyrotate relative to one another about the longitudinal axis of the spinalcolumn through a range of angles without causing the vertebral bodycontact surfaces to move away from one another along the longitudinalaxis. Once the axial rotation exceeds that range, the articulationsurfaces interfere with one another as the concave arcs move towardpositions in which they would be parallel to one another, and thedistance between the vertebral body contact surfaces increases withcontinued axial rotation as the concave arcs ride up against theiroppositely directed slopes. Thus, the articulation surfaces areconfigurable according to the present invention to permit normalphysiologic axial rotational motion of the adjacent vertebral bonesabout the longitudinal axis through a range of angles without abnormalimmediate axially opposite (or otherwise directed away from one another)movement, and to permit such axially opposite (or otherwise directedaway from one another) movement when under normal physiologic conditionsit should occur, that is, outside that range of angles.

[0033] In preferred embodiments where the constant radius of curvature Ais larger than the constant radius of curvature C, and the constantradius of curvature D is larger than the constant radius of curvature B,the articulation surfaces maintain point-to-point contact over a rangeof normal physiologic articulating movement between the adjacentvertebral bones. That is, through flexion, extension, lateral bending,and axial rotation, the articulation surfaces are in point-to-pointcontact with one another.

[0034] Preferably, the surface area dimensions of the articulationsurfaces are selected in view of the selected radii of curvature toprevent the edges of the saddle surfaces (particularly the edges of theconcave arcs) from hitting any surrounding anatomic structures, or otherportions of the opposing upper or lower element, before the limit of thenormal physiologic range of an attempted articulation is reached.

[0035] The novel features of the present invention, as well as theinvention itself, both as to its structure and its operation will beunderstood from the accompanying drawings, taken in conjunction with theaccompanying description, in which similar reference characters refer tosimilar parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIGS. 1-5 show an artificial disc implant of the present inventionin perspective, anterior, lateral, lateral cutaway, and posteriorcutaway views, respectively.

[0037]FIGS. 6-12 show an upper element of the artificial disc implant ofFIGS. 1-5 in perspective, bottom (looking longitudinally up), lateral,anterior, lateral cutaway, top (looking longitudinally down), andposterior cutaway views, respectively.

[0038]FIGS. 13-19 show a lower element of the artificial disc implant ofFIGS. 1-5 in perspective, top (looking longitudinally down), lateral,anterior, lateral cutaway, bottom (looking longitudinally up), andposterior cutaway views, respectively.

[0039]FIG. 20 shows a lateral cross-section view of the artificial discimplant of FIGS. 1-5 in extension.

[0040]FIG. 21 shows a posterior cross-section view of the artificialdisc implant of FIGS. 1-5 in lateral bending.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to the embodimentillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended, such alterations andfurther modifications in the illustrated device, and such furtherapplications of the principles of the invention as illustrated therein,being contemplated as would normally occur to one skilled in the art towhich the invention relates.

[0042] Referring now to FIGS. 1-5, an artificial disc implant 100 of thepresent invention is shown in perspective, anterior, lateral, lateralcutaway, and posterior cutaway views, respectively. The implant 100includes a first (e.g., upper) element 200 and a second (e.g., lower)element 300, each having an outwardly facing vertebral body contactsurface 202, 302, and each having an inwardly facing articulationsurface 204, 304. The elements 200, 300 are disposed as shown with thearticulation surfaces 204, 304 nested against one another, and thevertebral body contact surfaces 202, 302 facing away from one another.When the implant 100 is disposed in an intervertebral disc space in acervical spine, in this configuration and with the vertebral bodycontact surfaces 202, 302 engaged with respective adjacent vertebralbody endplates (not shown), the implant 100 enables the adjacentvertebral bones to move relative to one another in accordance withproper anatomical motion, as further described below.

[0043] Preferably, at least one (and more preferably both) of theelements 200, 300 has at least one long-term fixation structure (e.g.,flange 206, 306) having at least one feature (e.g., through hole 208 a,208 b, 308) for securing the element to an adjacent vertebral body. Forexample, the upper element 200 has an anterior flange 206 that extendsupwardly and has two through holes 208 a, 208 b, each of which accepts abone screw (not shown). And, for example, the lower element 300 has ananterior flange 306 that extends downwardly and has one through hole 308that accepts a bone screw (not shown). Once the elements 200, 300 aredisposed in the intervertebral space with the vertebral body contactsurfaces 202, 302 engaged with respective adjacent vertebral bodyendplates (not shown), securing of bone screws through the holes 208 a,208 b, 308 and into the anterior surfaces of the adjacent vertebralbones helps prevent the elements from becoming dislodged from, ordisplaced in, the intervertebral space. Preferably, the bore axes of thethrough holes 208 a, 208 b, 308 are angled toward the adjacent vertebralbody as shown.

[0044] Further preferably, at least one (and more preferably both) ofthe elements 200, 300 has at least one short-term fixation structure(e.g., spike 210 a, 210 b, 310 a, 310 b) for securing the element to anadjacent vertebral body (and more preferably to an adjacent vertebralbody endplate). For example, each of the elements 200, 300 has arespective pair of outwardly directed spikes 210 a, 210 b, 310 a, 310 b.Once the elements 200, 300 are disposed in the intervertebral space withthe vertebral body contact surfaces 202, 302 engaged with respectiveadjacent vertebral body endplates (not shown), the spikes 210 a, 210 b,310 a, 310 b dig into the adjacent vertebral body endplates under thecompression along the longitudinal axis of the spinal column, and thushelp prevent the elements from becoming dislodged from, or displaced in,the intervertebral space. Preferably, each of the spikes 210 a, 210 b,310 a, 310 b is sloped toward the vertebral body contact surface 202,302 and toward the posterior direction on its posterior side as shown,to facilitate ease of insertion of the implant 100 into theintervertebral space, and is either perpendicular to the vertebral bodycontact surface 202, 302 on its anterior side (as shown) or slopedtoward the vertebral body contact surface 202, 302 and toward theposterior direction on its anterior side (not shown), to help preventthe elements 200, 300 from anteriorly (or otherwise) slipping out of theintervertebral space.

[0045] More particularly, and referring now to FIGS. 6-12, the upperelement 200 of the artificial disc implant 100 shown in FIGS. 1-5 isshown in perspective, bottom (looking longitudinally up), lateral,anterior, lateral cutaway, top (looking longitudinally down), andposterior cutaway views, respectively. Further particularly, andreferring now to FIGS. 13-19, the lower element 300 of the artificialdisc implant 100 shown in FIGS. 1-5 is shown in perspective, top(looking longitudinally down), lateral, anterior, lateral cutaway,bottom (looking longitudinally up), and posterior cutaway views,respectively.

[0046] As introduced above, each of the elements 200, 300 has alongitudinally outwardly directed vertebral body contact surface 202,302. Preferably, each surface 202, 302 is shaped to conform to anendplate of an adjacent vertebral body (not shown) against which it isto be positioned. For example, inasmuch as a review of the relevantanatomy indicates that lower endplates of vertebral bones in thecervical spine each have a central concavity, it is preferable that thesurface 202 is curvate (and more preferably, domed as shown, orsemi-cylindrical), to conform to the central concavity. And, forexample, inasmuch as a review of the relevant anatomy indicates thatupper endplates of vertebral bones in the cervical spine are generallyflat, it is preferable that the surface 302 is flat, as shown. It shouldbe understood that the surfaces 202, 302 can be formed or can bedynamically formable to have these or other shapes that closely conformto the adjacent vertebral body endplate, without departing from thescope of the present invention.

[0047] Each vertebral body contact surface 202, 302 further preferablyhas an osteoinductive or osteoconductive feature. For example, eachsurface 202, 302 is preferably porous and/or roughened. This can beaccomplished by any manner known in the art, including, for example,grit blasting, porous coating, etching, burning, electrical dischargemachining, and sintered beading. While both surfaces 202, 302 arepreferably provided with such a feature, it should be understood thatonly one could have such a feature without departing from the scope ofthe present invention. Further, it should be understood that it is notnecessary for the entire surface to be so featured, but rather only aportion, some portions, or all of the surface can be so featured, orhave a variety of such features, without departing from the scope of thepresent invention.

[0048] Each vertebral body contact surface 202, 302 further preferablyhas the long-term fixation and short-term fixation structures describedabove and denoted by corresponding reference numbers on these FIGS.6-19.

[0049] As introduced above, the upper element 200 has a longitudinallyinwardly directed articulation surface 204. Preferably, as shown, thearticulation surface 204 includes a saddle surface that is defined by aconcave arc (denoted by reference numeral 212 on FIG. 10) that is sweptperpendicular to and along a convex arc (denoted by reference numeral214 on FIG. 12). As best seen in FIGS. 4, 5, 10, and 12, thearticulation surface 204 has a cross-section in one plane that forms aconcave arc 212, and a cross-section in another plane (perpendicular tothat plane) that forms a convex arc 214. The concave arc 212 has arespective substantially constant radius of curvature about an axisperpendicular to the one plane. The convex arc. 214 has a respectivesubstantially constant radius of curvature about an axis perpendicularto the other plane. Therefore, the articulation surface 204 forms asubstantially constant radii saddle-shaped articulation surface.

[0050] In this preferred embodiment, as indicated in FIG. 10, theconcave arc 212 has a substantially constant radius of curvature A aboutan axis perpendicular to the anterior-posterior plane. And, in thispreferred embodiment, as indicated in FIG. 12, the convex arc 214 has asubstantially constant radius of curvature B about an axis perpendicularto the lateral plane. Preferably, radius A is less than radius B, andmost preferably, radius A is 0.329 and radius B is 0.340. It should benoted, however, that the present invention is not limited to anyparticular dimension set, and further than in some embodiments of thepresent invention, radius A is equal to or greater than radius B.

[0051] Also as introduced above, the lower element 300 has alongitudinally inwardly directed articulation surface 304. Preferably,as shown, the articulation surface 304 includes a saddle surface that isdefined by a convex arc (denoted by reference numeral 312 on FIG. 17)that is swept perpendicular to and along a concave arc (denoted byreference numeral 314 on FIG. 19). As best seen in FIGS. 4, 5, 17, and19, the articulation surface 304 has a cross-section in one plane thatforms a convex arc 312, and a cross-section in another plane(perpendicular to that plane) that forms a concave arc 314. The convexarc 312 has a respective substantially constant radius of curvatureabout an axis perpendicular to the one plane. The concave arc 314 has arespective substantially constant radius of curvature about an axisperpendicular to the other plane. Therefore, the articulation surface304 also forms a substantially constant radii saddle-shaped articulationsurface.

[0052] In this preferred embodiment, as indicated in FIG. 17, the convexarc 312 has a substantially constant radius of curvature C about an axisperpendicular to the anterior-posterior plane. And, in this preferredembodiment, as indicated in FIG. 19, the concave arc 314 has asubstantially constant radius of curvature D about an axis perpendicularto the lateral plane. Preferably, radius C is less than radius D, andmost preferably, radius C is 0.280 inches and radius D is 0.401 inches.It should be noted, however, that the present invention is not limitedto any particular dimension set, and further than in some embodiments ofthe present invention, radius C is equal to or greater than radius D.Further in some embodiments, radii A, B, C, and D are of equaldimension.

[0053] Importantly, the substantially constant radii saddle shapedarticulation surfaces 204, 304 are configured and sized to be nestableagainst one another and articulatable against one another, to enableadjacent vertebral bones (against which the upper and lower elements200, 300 are respectively disposed in the intervertebral space) toarticulate in flexion, extension, and lateral bending. Moreparticularly, as best shown in FIGS. 1-5, the artificial disc implant100 of the present invention is assembled by disposing the upper 200 andlower 300 elements such that the vertebral body contact surfaces 202,302 are directed away from one another, and the articulation surfaces204, 304 are nested against one another such that the concave arc 212accommodates the convex arc 312 and such that the convex arc 214 isaccommodated by the concave arc 314. Either during or after suchassembly of the implant 100, the vertebral body contact surface 202 ofthe upper element 200 is fixed against a lower endplate of a superiorvertebral body (not shown), and the vertebral body contact surface 302of the lower element 300 is fixed against an upper endplate of aninferior vertebral body (not shown). As noted above, the preferablelong-term and short-term fixation structures on the elements 200, 300are useful for securing the elements 200, 300 to these adjacentvertebral bones.

[0054] Accordingly, movement of the adjacent vertebral bones relative toone another is permitted by the movement of the upper 200 and lower 300elements relative to one another. With regard to the articulationsurfaces 204, 304 being configured and sized to enable the adjacentvertebral bones to articulate in flexion, extension, and lateralbending, it is understood from the described geometry and positioning ofthe upper 200 and lower 300 elements once the implant 100 is assembledand implanted that in flexion and extension, the concave arcs (e.g.,212) of the upper element 200 ride on the convex arcs (e.g., 312) of thelower element 300 about a center of rotation (referenced as R3 on FIG.18) at the center of the circle defined by the convex arc 312. Thiscenter of rotation R3 is below the articulation surface 304. It isfurther understood from the described geometry and positioning of theupper 200 and lower 300 elements that in lateral bending, the concavearcs (e.g., 314) of the lower element 300 ride on the convex arcs (e.g.,214) of the upper element 200 about a center of rotation (referenced asR2 on FIG. 12) at the center of the circle defined by the convex arc214. This center of rotation R2 is above the articulation surface 204.During these articulations, the elements 200, 300 are maintained atsubstantially constant relative distraction positions, i.e, the elements200, 300 do not significantly move (if at all) in directions that aredirected away from one another (for example, do not move in opposingaxial directions from one another (e.g., along the longitudinal axis ofthe spine)). Accordingly, the present invention provides a pair ofarticulation surfaces 204, 304 that have a center of rotation above thesurfaces in one mode of motion (lateral bending), and below the surfacesin another (flexion/extension), consistent in these regards with anatural intervertebral joint in the cervical spine. Preferably, thearticulation surfaces 204, 304 are sized and configured so that therespective ranges of angles through which flexion/extension and lateralbending can be experienced are equal to or greater than the respectivenormal physiologic ranges for such movements in the cervical spine.While the present invention is not limited to any particular dimensions,a preferred embodiment has the following radii of curvature for theconvex arc 312 and the convex arc 214: C=0.280 inches and B=0.340inches. Such preferable radii of curvature provide the preferredembodiment with a flexion/extension range of plus or minus 7.5 degrees(total of 15 degrees), and a lateral bending range of plus or minus 7.5degrees (total of 15 degrees).

[0055] While the preferred embodiment is shown with concave arc 212having a larger constant radius of curvature A than the constant radiusof curvature C of convex arc 312 (for reasons that are described indetail below), and with concave arc 314 having a larger constant radiusof curvature D than the constant radius of curvature B of convex arc 214(for reasons that are described in detail below), it should beunderstood that the above described functionality can also be achievedusing other relative radii sizes. For example, flexion, extension, andlateral bending are also possible if the constant radius of curvature Aof concave arc 212 is otherwise non-congruent with (e.g., less than) oris congruent with (i.e., equal to) the constant radius of curvature C ofconvex arc 312, and/or if the constant radius of curvature D of concavearc 314 is otherwise non-congruent with (e.g., less than) or iscongruent with (i.e., equal to) the constant radius of curvature B ofconvex arc 214.

[0056] As noted above, it is preferable that, in addition to theflexion, extension, and lateral bending motions described above, theadjacent vertebral bones be permitted by the artificial disc implant 100to axially rotate relative to one another (e.g., about the longitudinalaxis of the spinal column), through a range of angles without moving inopposite (or otherwise directed away from one another) directions (e.g.,along the longitudinal axis) within that range. Preferably, thearticulation surfaces 204, 304 are accordingly configured and sized topermit such movement. Referring again to FIGS. 1-5, a preferredconfiguration is shown as an example, where the constant radius ofcurvature A of concave arc 212 is larger than the constant radius ofcurvature C of convex arc 312, and the constant radius of curvature D ofconcave arc 314 is larger than the constant radius of curvature B ofconvex arc 214. It is understood from the described geometry andpositioning of the upper 200 and lower 300 elements that, because of thespace, afforded by the differing radii, at the edges of the articulationsurfaces 204, 304, the upper 200 and lower 300 elements are able toaxially rotate relative to one another (e.g., about the longitudinalaxis) through a range of angles without causing the vertebral bodycontact surfaces 202, 302 to move in opposite (or otherwise directedaway from one another) directions (e.g., along the longitudinal axis).Once the axial rotation exceeds that range, the articulation surfaces204, 304 interfere with one another as the concave arcs 212, 314 movetoward positions in which they would be parallel to one another, and thedistance between the vertebral body contact surfaces 202, 302 increaseswith continued axial rotation as the concave arcs 212, 314 ride upagainst their oppositely directed slopes. Thus, the articulationsurfaces 204, 304 are configurable according to the present invention topermit normal physiologic axial rotational motion of the adjacentvertebral bones about the longitudinal axis of the spinal column througha range of angles without abnormal immediate axially opposite (orotherwise directed away from one another) movement, and to permit suchaxially opposite (or otherwise directed away from one another) movementwhen under normal physiologic conditions it should occur, that is,outside that range of angles. While the present invention is not limitedto any particular dimensions, a preferred embodiment has the followingradii of curvature: A=0.329 inches, B=0.340 inches, C=0.280 inches, andD=0.401 inches. Such preferable radii of curvate provide the preferredembodiment with a longitudinal axial rotation range of plus or minus 3degrees (total of 6 degrees) before oppositely directed movement of thearticulating surfaces occurs.

[0057] It should be noted that in the preferred embodiment, and in otherpreferable embodiments where the constant radius of curvature A ofconcave arc 212 is larger than the constant radius of curvature C ofconvex arc 312, and the constant radius of curvature D of concave arc314 is larger than the constant radius of curvature B of convex arc 214,the articulation surfaces 204, 304 maintain point-to-point contact overa range of normal physiologic articulating movement between the adjacentvertebral bones. This is illustrated in FIGS. 4, 5, 20, and 21. Moreparticularly, it is understood from the described geometry andpositioning of the upper 200 and lower 300 elements that throughflexion, extension, lateral bending, and axial rotation, thearticulation surfaces 204, 304 are in point-to-point contact with oneanother as they are in FIGS. 4 and 5. FIGS. 20 and 21 are provided toshow the implant 100 in extension and lateral bending, respectively, tofurther illustrate this preferable feature.

[0058] It should further be noted that in addition to the radii ofcurvature dimensions of the articulation surfaces 204, 304 beingrelevant to a configuration and sizing of the articulation surfaces 204,304 permitting normal physiologic flexion, extension, lateral bending,and axial rotation movements of the adjacent vertebral bones, thesurface area dimensions are also relevant, particularly in relation tothe selected radii of curvature. More particularly, in order to providea range of relative angulation that is within the normal physiologicrange of the cervical spine, not only must the selected radii ofcurvature be suitable as described above, but also and accordingly thesurface area of the saddle surfaces should be of a dimension that, giventhe selected radii of curvature, prevents the edges of the saddlesurfaces (particularly the edges of the concave arcs (e.g., 212 ande.g., 314)) from hitting any surrounding anatomic structures, or otherportions of the opposing element (200 or 300), before the limit of thenormal physiologic range of the attempted articulation is reached. Asshown, one or both of the inwardly facing surfaces of the upper 200 andlower 300 elements can be tapered inwardly before presenting itsarticulation surface (204 or 304), to ensure a suitable surface areadimension to prevent such interference. While the present invention isnot limited to any particular surface area dimensions, the illustratedpreferred embodiment has a surface area of articulation surface 204equal to 0.146 square inches, and a surface area of articulation surface304 equal to 0.153 square inches.

[0059] Further preferably, the articulation surfaces 204, 304 are formedof cobalt-chrome that is polished to provide a smooth bearing surface.It should be understood that the articulation surfaces 204, 304, whilepreferably formed of cobalt-chrome, can be additionally or alternativelyformed of other metals, such as, for example, stainless steel and/ortitanium, or of non-metals, such as, for example, polyethylene and/orceramic materials (e.g., alumina and zirconia), or of any other suitablematerial without departing from the scope of the present invention.

[0060] It should be noted that while the present invention isillustrated and described as an artificial disc implant for use in thecervical spine, the artificial disc implant of the present invention canbe adapted for use in any other portion of the spine without departingfrom the scope of the present invention.

[0061] While the particular prostheses for the cervical intervertebraljoint of the spine as herein shown and disclosed in detail are eachfully capable of obtaining the objects and providing the advantagespreviously stated, it shall be understood that these variations aremerely illustrative of the presently preferred embodiments of theinvention and that no limitations to the scope of the present inventionare intended to be inferred from the details of the construction ordesign herein shown.

1. An apparatus for replacing at least a portion of an intervertebraldisc in a spinal column, comprising: a first member having a firstvertebral contact surface for engagement with an endplate of a firstvertebral bone in the spinal column, and having a first articulationsurface that is defined by a plurality of concave arcs each having arespective radius of curvature about a corresponding axis substantiallyperpendicular to an anterior-posterior plane of the spinal column, and aplurality of convex arcs each having a respective radius of curvatureabout a corresponding axis substantially perpendicular to a lateralplane of the spinal column; and a second member having a secondvertebral contact surface for engagement with an endplate of a secondvertebral bone in the spinal column, and having a second articulationsurface that is defined by a plurality of convex arcs each having aradius of curvature about a corresponding axis substantiallyperpendicular to the anterior-posterior plane of the spinal column, anda plurality of concave arcs each having a radius of curvature about acorresponding axis substantially perpendicular to the lateral plane ofthe spinal column, wherein: an intervertebral disc space is definedsubstantially between the first and second endplates of the first andsecond vertebral bones, and the radii of curvature of the first andsecond articulation surfaces are sized such that the first and secondarticulation surfaces engage one another when the first and secondmembers are disposed in the intervertebral disc space to enable thefirst and second vertebral bones to articulate in at least one offlexion, extension and lateral bending.
 2. The apparatus of claim 1,wherein at least one of: (i) the axes perpendicular to theanterior-posterior plane of the spinal column are substantially coaxial;and (ii) the axes perpendicular to the lateral plane of the spinalcolumn are substantially coaxial.
 3. The apparatus of claim 1, whereinat least one of: (i) the axes perpendicular to the anterior-posteriorplane of the spinal column lie in a plane that is substantiallyperpendicular to the anterior-posterior plane; and (ii) the axesperpendicular to the lateral plane of the spinal column lie in a planethat is substantially perpendicular to the lateral plane.
 4. Theapparatus of claim 1, wherein: at least one of the concave arcs of thefirst articulation surface has a radius of curvature A about a firstaxis substantially perpendicular to the anterior-posterior plane of thespinal column, and at least one of the convex arcs of the firstarticulation surface has a radius of curvature B about a first axissubstantially perpendicular to the lateral plane of the spinal column;and at least one of the convex arcs of the second articulation surfacehas a radius of curvature C about a second axis substantiallyperpendicular to the anterior-posterior plane of the spinal column, andat least one of the concave arcs of the second articulation surface hasa radius of curvature D about a second axis substantially perpendicularto the lateral plane of the spinal column.
 5. The apparatus of claim 1,wherein: the concave arcs of the first articulation surface are ofsubstantially the same radius of curvature, each generally about acorresponding axis substantially perpendicular to the anterior-posteriorplane of the spinal column; the convex arcs of the first articulationsurface are of substantially the same radius of curvature, eachgenerally about a corresponding axis substantially perpendicular to thelateral plane of the spinal column; the convex arcs of the secondarticulation surface are of substantially the same radius of curvature,each generally about a corresponding axis substantially perpendicular tothe anterior-posterior plane of the spinal column; and the concave arcsof the second articulation surface are of substantially the same radiusof curvature, each generally about a corresponding axis substantiallyperpendicular to the lateral plane of the spinal column.
 6. Theapparatus of claim 1, wherein: the concave arcs of the firstarticulation surface are of differing radii of curvature, each generallyabout a corresponding axis substantially perpendicular to theanterior-posterior plane of the spinal column; and the convex arcs ofthe second articulation surface are of differing radii of curvature,each generally about a corresponding axis substantially perpendicular tothe anterior-posterior plane of the spinal column.
 7. The apparatus ofclaim 6, wherein the convex arcs of the second articulation surface areof differing radii of curvature that correspond with the differing radiiof curvature of the concave arcs of the first articulation surface. 8.The apparatus of claim 6, wherein: the anterior-posterior plane and thelateral plane intersect along a longitudinal axis of the intervertebraldisc space when the apparatus is disposed in the intervertebral discspace; one or more of the radii of the concave arcs of the firstarticulation surface reduce as distances of origins of the radii fromthe anterior-posterior plane along the corresponding axes increase; andone or more of the radii of the convex arcs of the second articulationsurface reduce as distances of origins of the radii of the convex arcsof the second articulation surface from the anterior-posterior planealong the corresponding axes increase.
 9. The apparatus of claim 8,wherein the one or more of the radii of the convex arcs of the secondarticulation surface reduce in correspondence with the radii of theconcave arcs of the first articulation surface.
 10. The apparatus ofclaim 6, wherein: the convex arcs of the first articulation surface areof substantially the same radius of curvature, each generally about acorresponding axis substantially perpendicular to the lateral plane ofthe spinal column; and the concave arcs of the second articulationsurface are of substantially the same radius of curvature, eachgenerally about a corresponding axis substantially perpendicular to thelateral plane of the spinal column.
 11. The apparatus of claim 1,wherein: the convex arcs of the first articulation surface are ofdiffering radii of curvature, each generally about a corresponding axissubstantially perpendicular to the lateral plane of the spinal column;the concave arcs of the second articulation surface are of differingradii of curvature, each of the concave arcs of the second articulationsurface being generally about a corresponding axis substantiallyperpendicular to the lateral plane of the spinal column.
 12. Theapparatus of claim 1, wherein the concave arcs of the secondarticulation surface are of differing radii of curvature that correspondwith the differing radii of curvature of the convex arcs of the firstarticulation surface.
 13. The apparatus of claim 11, wherein: theanterior-posterior plane and the lateral plane intersect along alongitudinal axis of the intervertebral disc space when the apparatus isdisposed in the intervertebral disc space; one or more of the radii ofthe convex arcs of the first articulation surface reduce as distances oforigins of the radii from the lateral plane along the corresponding axesincrease; and one or more of the radii of the concave arcs of the secondarticulation surface reduce as distances of origins of the radii of theconvex arcs of the second articulation surface from the lateral planealong the corresponding axes increase.
 14. The apparatus of claim 13,wherein the one or more of the radii of the concave arcs of the secondarticulation surface reduce in correspondence with the radii of theconvex arcs of the first articulation surface.
 15. The apparatus ofclaim 11, wherein: the concave arcs of the first articulation surfaceare of substantially the same radius of curvature, each generally abouta corresponding axis substantially perpendicular to theanterior-posterior plane of the spinal column; and the convex arcs ofthe second articulation surface are of substantially the same radius ofcurvature, each generally about a corresponding axis substantiallyperpendicular to the anterior-posterior plane of the spinal column. 16.The apparatus of claim 1, wherein the first and second articulationsurfaces are sized and shaped to engage one another when the first andsecond members are disposed in the intervertebral disc space to enablethe first and second vertebral bones to at least axially rotate relativeto one another through a range of angles.
 17. The apparatus of claim 1,wherein the first and second articulation surfaces are sized and shapedto engage one another when the first and second members are disposed inthe intervertebral disc space to enable the first and second vertebralbones to axially rotate relative to one another through a range ofangles without substantially displacing the first and second vertebralbones away from one another.
 18. The apparatus of claim 17, wherein thefirst and second articulation surfaces are sized and shaped to achievesubstantial point-to-point contact relative to one another when in atleast some positions of flexion, extension, lateral bending, and/oraxial rotation.
 19. The apparatus of claim 17, wherein the range ofangles is about plus/minus three degrees from a resting position. 20.The apparatus of claim 17, wherein the first and second articulationsurfaces are sized and shaped such that the first and second vertebralbones are displaced away from one another at axial rotations outside therange of angles.